There are many different types of lapping or polishing machines including those specifically designed or adapted for reducing the thickness of semiconductor wafers by removing various layers of material added during manufacturing. One such device and its use is described in U.S. Pat. No. 4,793,895 assigned to the assignee of the present invention. This patent disclosed an end point detector in which a plurality of isolated electrodes are embedded in the lapping pad on the rotating polishing table of the machine, and means are provided for causing and monitoring current flow between the spaced isolated electrodes with the current flow being indicative of the amount of conductive material on the exposed surface layer.
While this machine does perform with reasonable effectiveness, nevertheless there are certain drawbacks, the principal of which is that all of the electronics, including the current inducing and measuring devices must be contained on the rotating table. Hence, the measurements must be transferred from the rotating table to the surrounding monitoring and control equipment. The transfer is achieved by means of either an infra-red transmitter and sensor or some type of slip ring arrangement. With these types of transmissions of data from a rotating member to a stationary member there is occasionally a problem with the accuracy of transmission or the reception of information, due to various types of potential interference with either the infra-red transmitter or sensor, or with electrical or mechanical contact problems in the rotating slip rings.
In any event, it is desirable to provide for the measurement of the end point to be directly recorded on the stationary or non-rotating portion of the equipment without the necessity of transferring data from a rotating member to a stationary member.
According to the present invention, this is accomplished by utilizing an impedance detector which is mounted on the stationary or non-rotating portion of the lapping or polishing machine; and, on the rotating table, means are provided to change the impedance of the detector by perturbing a magnetic flux generated on the stationary portion of the equipment, which change is a function of the amount of material which has been removed from the surface of the wafer. The end point measurement is accomplished without physical contact between the rotating table and the stationary portion of the polishing or lapping machine.
Many prior art references teach various uses of a variety of types of magnetic and electrical coils and circuits for different detection schemes. For example, Myers, R.J. and Rankin, C.J., Handbook of Electronic Control Circuits, 1959, Pages 167-168, teaches the monitoring of current in a coil of an electromagnet to detect variations in any current caused by physical flaws in a conductor passed through a magnetic gap. IBM Technical Disclosure Bulletin dated Sep., 1966, at Pages 358 and 359, shows the use of an electromagnetic coil as a monitor of variations in an external passive conductor which passes through the gap of a coil. U.S. Pat. No. 4,804,912 shows a distance monitor which measures changes in the magnetic coupling between an electromagnetic coil and the surface of a conductive work piece and a grinding machine. In this reference, the active coil acts both as a stimulus for and a detector of the changes caused by a free moving passive field perturbing member and a grinding component. A similar application is described in the article "Magnetic Head Lapping Method" by F.W. Hahn, Jr. in IBM Technical Disclosure Bulletin dated Apr., 1974, Page 3509 where the signal is read by a magnetic head monitored during the lapping of a gap-forming layer to determine the head transducing gap. A recorded signal in the lapping tape is used to provide the flux perturbing signal.
None of these references suggest a remote impedance detector of the type provided according to this invention.